The present invention generally relates to an electric power-generation system and more particularly, to a DC-AC converting arrangement for a photovoltaic system.
For a photovoltaic system which obtains an electric power, for example, from sun light, there has been known a construction which is so arranged as shown in FIG. 1. An electromotive force produced by a solar cells array S is led, through a charge-discharge control unit D, to a storage battery V and a DC-AC converter C where it is converted from DC current to AC current so as to be subsequently fed to a load L through a switching device F. The charge-discharge control unit D has a control function for properly changing over between the charging and discharging state of the storage battery V, while the switching device F is adapted to selectively connect a commercial power supply system P or the photovoltaic system as a power supply source to the load L.
In the conventional photovoltaic system as described above, the DC-AC converter (inverter) C has output-efficiency characteristics as shown in FIG. 3, in which the efficiency is largely varied by the output power, with a marked deterioration of efficiency when in a state of low output. This is attributable to the fact that, even upon lowering of an output power, loss that occurs in a control and in various elements, within the DC-AC converter is not reduced in proportion thereto. Accordingly, in a conventional photovoltaic system, when it becomes inevitable to supply power at a low load factor, the electric power generated by the solar cells array is to be supplied from the DC-AC converting arrangement under a state with a very low efficiency, thus resulting in a wasteful loss of power. In the diagram of FIG. 3, the normal operating range of the DC-AC converter is represented by a symbol W. Moreover, the DC-AC converting arrangement is continuously operated under no load, even when the load becomes zero, and thus, the power produced by the photovoltaic system is undesirably consumed at the control circuit and at the various other elements.